One of many measuring methods for ascertaining fill level in a container is the travel-time measuring method. In the travel-time measuring method, for example, microwaves, or radar waves, are transmitted as measuring signals via an antenna apparatus, and the echo waves reflected on the surface of the medium are received back, following a distance-dependent travel time. From half of the travel time, the fill level of the medium in a container can then be calculated. The echo curve represents, in such case, received signal amplitude as a function of time, with each measured value of the echo curve corresponding to the amplitude of an echo signal reflected on a surface at a certain distance. The travel time measuring method is divided essentially into two detection methods, these being the time-difference measurement method, which ascertains the time needed for a broadband wave signal-pulse to travel over a path length, and the other, widely used detection method involving determination of the sweep frequency difference between the transmitted, frequency modulated, high frequency signal and the reflected, received, frequency modulated, high frequency signal (FMCW—Frequency-Modulated Continuous Wave).
In the following, no limitation is made to a specific detection method.
In process measurements technology, planar, array-type antennas have been used for a long time, since these, among other things, also radiate in special modes, e.g. the TE 01 mode. These modes have, for fill level measurements in containers, the advantage that they propagate almost uninfluenced by the container walls.
Such embodiments of planar antennas are disclosed in German patent application DE 101 18 009 A1 and European patent EP 1 083 413 B1.
A disadvantage of planar antennas is that they are most often oriented in a process measurement structure such that their planar surfaces are orthogonal to the gravitational field of the Earth, or, in other words, parallel to the surface of the fill substance. Given a temperature difference between the fill substance and the planar antenna, volatile constituents of the fill substance being measured condense on the planar antenna, and drops of the condensate form. This drop formation of the condensed fill substance changes the radiation characteristic of the antenna and measurements of fill level of a fill substance in a container are no longer correct. Since condensation of the evaporating fill substance, or medium, on the planar antenna can not be avoided, attempts have been made, based on structural measures, to overcome the adhesion forces, or surface tension, of the condensed medium on the material surface of the planar antenna. An embodiment of a planar antenna adapted in this way is described in U.S. Pat. No. 6,684,697, wherein the planar antenna is arranged tilted with respect to horizontal H by an angle a. By this tilted positioning, the force of gravity has also a force component parallel to the surface of the planar antenna, whereby the condensate, driven by this added force component, runs together to form larger drops, which then drop off at a given location. The change of the wavefront, or radiating direction, of the radiative lobe by the tilted position of the planar antenna is compensated by differing phase control for the antenna element rows. Additionally, disclosed in the U.S. Pat. No. 6,629,458 B1 is an embodiment of a planar antenna, in which, in front of the planar antenna, a filled cone is used as an antenna protective structure (radome). The filler is a dielectric, thermally insulating material, which has the same effect as described above, that the condensate can drop off of the surface of the antenna.
There are different types of planar antennas, which differ on the basis of their effects, embodiments and manufacture. For example, microstrip-patch-antennas have a series of notable advantages as regards small space requirement, simple manufacture, and low weight. A patch antenna is composed, in the simplest case, of a rectangular metal area (patch), which is mounted on a dielectric substrate above a conductive base. There is a multitude of possible shapes in use for special applications, including circular, elliptical, triangular and annular, patch radiators. The different forms serve, for example, for increasing bandwidth of the transmitted high frequency signals or for excitation of different modes.
The simplest kind of excitation occurs, for example, with the help of strip conductors, which are placed in the same plane as the patch-radiator antenna element. This kind of excitation has the advantage of simple and rapid manufacture. However, it has serious disadvantages, since the feeding network and the radiating antenna elements place very different requirements on the substrate material. For this reason, one-layer microstrip patch antennas with still acceptable radiative properties have only a small bandwidth. Help in this regard is provided by the use of multi-layer structural planes, where radiator and feeding network are arranged on different support structure layers, or substrate plies. The substrates, or support structures, for the feeding network and patch radiator antenna elements can then be optimally selected independently of one another. By this multilayer construction, however, an electrical connection between feeding network- and radiator-planes becomes necessary. One possibility is direct galvanic connection in the form of coaxial vias. Another, more elegant option is offered by antenna structures coupled via a coupling aperture. The coupling between feeding network and antenna elements occurs by narrow coupling apertures in the shared metallizing for grounding. On the basis of the slit coupling, the feeding network is completely shielded by the areal ground, which leads to very good radiative characteristics as regards side lobe levels, as well as cross polarization.
Embodiments of multi-layer, planar antennas of a glass ceramic are disclosed in U.S. Pat. No. 6,145,176 and in Published International Patent application WO 02/09232 A1. As also indicated in these documents, it is advantageous to build the multi-layer support structure on the basis of a glass ceramic which can be sintered at low temperatures (<1000° C.) (LTCC—Low Temperature Cofired Ceramics). This method, in which a plurality of thin layers of ceramic material in unfired state and structured metal structures are interfacially laminated together is very easily put into practice for a highly integrated building of a high frequency component. The planar, laminated, ceramic stack with the metal structures is fired at low temperatures, for providing its final strength.